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Description of Niche
Lichens are unique organisms that consists of a symbiotic relationship between fungal filaments and algae or cyanobacteria. Algae and cyanobacteria are known as photobionts, while the fungal component of lichen is called a mycobiont. Algae are photosynthetic and provide the fungus with carbohydrates which can be used to produce energy. Cyanobacteria on the other hand, fix nitrogen or carbon which the fungal can use to synthesize amino acids and carbohydrates respectively. The fungus can also be considered a parasite because it provides no dehydration protection or nutrients for the photobionts.
Lichens can inhabit a diverse environment ranging from deserts and tundras all over the world covering an estimated 8% of all land. Lichens can be found in deserts such as Saudi Arabia, Australia and North America. In these environments lichens are situated on tree barks, leaves, ground, and rocks.
As stated earlier lichens come in many form, each with different roles and adaptations that suit their environment. Many of these lichens can occupy the same niche but over time the lichen that has the best adaptations for the environment will have a higher growth rate and a lower death rate, and out live the poorly suited lichen. Lichens can even overgrow and kill mosses that are in their niche by excreting toxins. Lichens are also known to produce useful antibodies and dyes that have been harnessed by humans.
Lichens can survive in extreme desert heat. In the desert Lichens can survive up to -60 º to 55 º Celsius which is -76 to 131º Fahrenheit. The water content for lichens can vary from 2%-300% dry weight. The low water content lichens are ideal for desert rock lichens because deserts are known for their miniscule or non-existent precipitation. Due to the lack of precipitation, lichens obtain most of their water from the atmosphere and morning dews. When not enough water is obtained from the atmosphere the lichen turns off metabolic processes and go dormant, which can last several years. However when a small amount of water is obtained from morning dews the lichen comes out of its dormant stage and resumes physiological activities. Compared to cyanobacteria, algae are more adaptive at low water levels and can occupy desert niches better.
Since Lichens are composed of a symbiotic relationship between fungus and photosynthetic algae there can be many combinations of lichens. It is estimated that 85 percent of the photobiont are eukaryotic algae while 10 percent are cyanobacteria and 5 percent are a combination of cyanobacteria and eukaryotic algae. Some examples of alga are trebouxia, Pseudotrebouxia, and Trentepohlia while some examples of cyanobactera are Nostoc and Scytonema. The fungi that are in the symbiotic relationship can be Ascomycota, Basidiomycota, or Conidial fungi.
The relationship between the fungal and the alga or cyanobacteria can be considered to be parasitic because the fungal benefits from the photobionts but the photobionts do not benefit from the fungal. Studies using radioactively labeled carbon have shown that photobionts retain only 20% of the carbon they fix, while they transport 80% of the fixed carbon the fungus. Algae release the carbon they fix in the form of glucose, while cyanobacteria release carbons in the form of polyhydric alcohols. These forms of carbon can then move on to provide energy for the photobionts and the mycobionts.
Cyanobacteria, particularly Nostoc, are unique from alga because along with fixing carbon they also fix nitrogen into ammonia. Most of the ammonia is actually used by the fungus to synthesis necessary amino acids
Changes to Environment
Most lichens change the diverse environment they live in. They can form fruticose which looks like shrubs, foliose which are like leaves, crustose which are like a crust on the surfaces they are in contact with. Lichens are also known to weather rocks through chemical and physical methods. Chemical weathering of rocks is due to the secretion of oxalic acid converted from insoluble carbonates and silicates. Physical weathering of rocks is done by contracting and swelling of the body of the lichen. Swelling of the body of the lichen happens during wet periods, while contracting happens during the dry periods of the lichen. The changes in size of the lichen help those that are embedded in cracks of rocks to break the rock apart.
On June 6, 2005, U.S reserachers
Hawksworth, David L. “The Nature of Lichens” Encyclopedia of Life Science. University of California, San Diego, La Jolla, CA.
22 Jan. 2002. <http//mrw.interscience.wiley.com>
Douglas, Angela E. “Algal Symbioses” Encyclopedia of Life Science. University of California, San Diego, La Jolla, CA. 25 April. 2002.
Edited by [Priya Patel, Fatima Khan, Luanne Cabiling], students of Rachel Larsen